The present invention relates to novel resin compositions and to improved methods to prepare monomers to produce such resins and other valuable compounds. More particularly, it relates to resins derived from N-(substituted) maleimide monomers, the resins having excellent heat resistance, per se, and the ability to enhance heat resistance when mixed with other bulk resins, and to an improved method for the preparation of such maleimide monomers using judiciously selected polar solvents.
A variety of N-alkyl and N-aryl maleimides have been used to improve the heat stability of homo- and especially co-polymers prepared from vinyl monomers. Typically, the bulk resins comprise, ABS (poly-(acrylonitrile-co-butadiene-co-styrene)) or a polyblend of poly-(acrylonitrile-co-butadiene) and poly-(styrene-co-acrylonitrile); PVC (poly(vinyl chloride); SAN (poly(styrene-co-acrylonitrile)); PMMA (poly-(methyl methacrylate)); and the like. The maleimides can be copolymerized with other monomers such as acrylonitrile, butadiene, styrene, methyl methacrylate, vinyl chloride, vinyl acetate, and many other comonomers. A more preferred practice in the industry is to produce copolymers of maleimides with other monomers such as styrene and optionally acrylonitrile and to blend these with ABS and SAN resins. In any event, the polymer compositions are adjusted so that the copolymers are fully compatible with the bulk resins, e.g., ABS and/or SAN) as shown by the presence of a single glass transition point, T.sub.g, as determined by differential scanning calorimetry (DSC). Maleimide-containing aromatic vinyl resins prepared by suspension polymerization in the presence of organic peroxides and/or azo compounds, and their use in the manufacture of parts for automobiles, electrical and electronic machinery and appliances, because of their good heat resistance, impact resistance and moldability, are described, for example, in Japanese Patent Publication Nos. J02004810, Jan. 9, 1990; J62288655, Dec. 15, 1987; and J02147610, Jun. 6, 1990; and also in Park, et al., U.S. Pat. No. 5,028,651.
Among the numerous maleimides described in the literature, N-phenylmaleimide (NPMI) appears to be the monomer of choice so far. NPMI shows excellent properties as a material to improve heat resistance. The major drawbacks are its relatively low solubility in common organic solvents, high toxicity and deep yellow color which imparts dark color to the finished molded articles. Thus, related compounds without such drawbacks would be distinctly advantageous. It does not appear that N-(di(lower)alkylphenyl) maleimides, and especially N-(2,3-dimethylphenyl) maleimide, although known, have ever been used commercially in place of NPMI. Furthermore, homopolymers and copolymers of the known compound N-(2,3-dimethylphenyl) maleimide have never apparently been described although they can be said to have been described generically. That is, the above mentioned Japanese Publications describe the preparation of copolymers of alkyl substituted aryl maleimides, but N-(2,3-dimethylphenyl) maleimide is not included in the list. N-(2,3-dimethylphenyl) maleimide is described for use as an industrial microbiocide and as a human anti-fungal agent in a number of publications, among which are Japanese Patent Publication Nos. J02240002, Sep. 25, 1990; J57016804, Jan. 28, 1982; British Patent No. 1533067, Nov. 22, 1978; and "Nippon Kagaku Kaishi" (9), 1616-19 (1989).
Applicants have now prepared this N-(2,3-dimethylphenyl) maleimide and discovered that it is a pale yellow solid and appears to be less toxic than the phenyl analog (NPMI). It has higher solubility than the phenyl analog; and, surprisingly, its novel homo- and co-polymers are easily compatible with SAN and with other polymers. As an important part of this work, applicants have also discovered an improved process for the preparation of N-(substituted) maleimides, which depends on the use of specific aprotic and protic polar solvents. The non-polar solvent, xylene, seems to have been the solvent of choice until now (see, for example, the above-mentioned Japanese Patent Publication No. J02240002; and GB No. 1533067, both of which use xylene). While not intending to be bound by any theory of operation, it appears that careful selection of the solvent reduces the amount of trans-amic acid formed in the first stage of reaction between maleic anhydride and the amines. Because the cis-isomer is preferentially produced and the trans-isomer does not cyclize, but the cis-isomer does, high yields of the desired maleimide are surprisingly obtained.